"What shape, then, must the mutual restraints take when co-operation begins? or rather, what, in addition to the primary mutual restraints already specified, are those secondary mutual restraints required to make co-operation possible? * * * * The reply will be made clearer if we take the successive forms of co-operation in the order of ascending complexity. We may distinguish as homogeneous co-operation (1) that in which like efforts are joined for like ends that are simultaneously enjoyed. As co-operation that is not completely homogeneous we may distinguish (2) that in which like efforts are joined for like ends that are not simultaneously enjoyed. A co-operation of which the heterogeneity is more distinct is (3) that in which unlike efforts are joined for like ends. And lastly comes the decidedly heterogeneous co-operation, (4) that in which unlike efforts are joined for unlike ends."[14] The social attainment reaches a full development in the last mentioned case. "Only under voluntary agreement then, no longer tacit and vague, but overt and definite, can co-operation be harmoniously carried on when division of labour becomes established. And, as in the simplest co-operation, where like efforts are joined to secure a common good, the dissatisfaction caused in those who, having expended their labours, do not get their share of the good, prompts them to cease co-operating; as in the more advanced co-operation, achieved by exchanging equal labours of like kind expended at different times, aversion to co-operate is generated if the expected equivalent of labour is not rendered; so in this developed co-operation, the failure of either to surrender to the other that which was avowedly recognized as of like value with the labour or product given, tends to prevent co-operation by exciting discontent with its results. And, evidently while antagonisms thus caused impede the lives of the units, the life of the aggregate is endangered by diminished cohesion." "But now we have to recognise the fact that complete fulfilment of these conditions, original and derived, is not enough. * * * * If no one did for his fellows anything more than was required by strict performance of contract, private interests would suffer from the absence of attention to public interests. The limit of evolution of conduct is consequently not reached, until, beyond avoidance of direct and indirect injuries to others, there are spontaneous efforts to further the welfare of others." The point brought out here is the social pressure of the society upon the individual, so as to ensure that the actions of the individual primarily are not inimical to its welfare, and secondarily are subservient to its welfare. But, of course, since society is composed of individuals, this pressure must not be of such a character as to be destructive of the welfare of the individuals of which the society is composed, for that would militate against its own objects.